Implement `valid_period` extension

[alejoe91]

We need an extension to annotate units with valid periods.
The extension data is quite simple:

• a list of [start_frame, end_frame] for each unit
• list of list if multi-segment

[alejoe91]

@samuelgarcia suggests a structured array with:

(segment_index, unit_index, start_frame, end_frame)

which is great because it is a numpy array and it's automatically handled by the BaseExtensions